Add Rails 6.0 w/ Ruby 3.0 to CI matrix again
Add Rails 6.0 w/ Ruby 3.0 to CI matrix again

Rails 6.0.4 has been released and includes a fix for our CI.
https://weblog.rubyonrails.org/2021/6/15/Rails-6-0-4-has-been-released/
https://github.com/rails/rails/pull/39697

mongomapper/mongomapper